Position-measuring devices which provide an absolute position value are being used increasingly in automation technology. Certain disadvantages of what are termed incremental position-measuring devices are thereby eliminated such as, for example, the necessity of carrying out a homing procedure after the switch-on operation in order to find a reference position which is used as reference point for the further position measuring by counting graduation marks.
Primarily serial data interfaces are used for transmitting absolute position values, since they make do with only a few data-transmission lines, and nevertheless, have high data-transmission rates. Particularly advantageous here are what are called synchronous serial interfaces, which have one unidirectional or bidirectional data line and one clock line. Data packets are transmitted via the data line in synchronism with a clock signal on the clock line. A multitude of standard interfaces have gained acceptance in automation technology; for example, popular representatives for synchronous serial interfaces are the EnDat interface of HEIDENHAIN, and a further is known under the name SSI. In addition, asynchronous serial interfaces such as Hiperface are also prevalent.
The SSI interface is described in European Published Patent Application No. 0 171 579. It is a synchronous serial data interface having one unidirectional data line and one unidirectional clock line. Position values from a position-measuring device are read out here in synchronism with a clock signal on the clock line.
On the other hand, European Patent No. 0 660 209 describes the fundamentals of the EnDat interface. It is likewise a synchronous serial interface which, however, besides the unidirectional clock line, has a bidirectional data line. It is thereby possible to transmit data in both directions—from the sequential electronics to the position-measuring device and from the position-measuring device to the sequential electronics. The data is transmitted in synchronism with a clock signal on the clock line here, as well.
German Patent No. 197 01 310 describes a device for transmitting data between a sensing element in the form of a position-measuring system, and a processing unit. By transmitting a reference signal on one of the signal-transmission lines, via which data is transmitted between the sensing element and the processing unit, it is possible to switch the position-measuring system to various operating modes.
Standardized interfaces offer the advantage that measuring devices which are equipped with such an interface may be connected directly to sequential electronics, e.g., a machine-tool control. However, a disadvantage for the measuring-device manufacturer is that it must offer the measuring devices with various standard interfaces in order to be able to provide solutions for sequential electronics that are already equipped with a specific interface. The result is a great variety of versions, which requires high expenditure for product revisions and complicates stockkeeping considerably.
Japanese Published Patent Application No. 8-185591 describes an absolute position-measuring device which supports a plurality of transmission formats. The transmission format is selected via a selection signal, which is supplied to the position-measuring device via additional lines from the sequential electronics. The requirement to make additional lines available increases the cabling expenditure considerably, and is therefore undesirable. In addition, this solution is inflexible, since the transmission format must be established manually.